A trench isolated silicon layer in a semiconductor substrate is commonly required. For example, where it is desired to electrically insulate discrete areas of a silicon layer one from the other trenches filled with electrically insulating filling material are formed in the silicon layer for electrically insulating the respective discrete areas. Commonly, such layers requiring trench filled isolation may be a silicon layer of a silicon on insulator (SOI) structure. A buried insulating layer, typically, an oxide layer is located under the silicon layer, and the silicon layer and oxide layer are in turn supported on a handle layer. Where trench filled electrical isolation is required, the trench or trenches are formed through the silicon layer to the buried insulating layer. After forming of the trenches, the trenches are filled with an appropriate electrical insulating material, which may be an oxide or silicon nitride. The oxide or silicon nitride are generally deposited by a chemical vapour deposition process. Alternatively, in order to strengthen the SOI structure, the trenches may be initially partially filled with an electrical insulating material, such as for example, an oxide, which would form a lining layer in the trench. The remaining part of the trench would then be filled with polysilicon. Typically, the oxide is deposited in one or more layers in the trench which line the base which is formed by the buried oxide layer, and the side wells of the trench. The remaining unfilled portion of the trench is then filled with polysilicon. Typically, the polysilicon is deposited by a chemical vapour deposition process.
Whether the trenches are entirely filled with a single filling material, for example, oxide or silicon nitride, or partially filled with oxide or silicon nitride and then with polysilicon, problems arise in the filling of such trenches.
As the filling material, be it oxide silicon nitride or polysilicon, or a combination of such materials are being sequentially deposited, a layer of the filling material of progressively increasing depth forms on the base of the trench and on the side walls of the trench, as well as on the surface of the silicon layer through which the trench is formed into the silicon layer. Unfortunately, the depth of the layers of the filling material formed on the base and the side walls of the trench, and on the surface of the silicon layer do not increase in depth at a uniform rate. Typically, the depth of the filling material on the surface of the silicon layer increases at a greater rate then the depth of the filling material on the side walls of the silicon layer. This, thus, leads to an effect commonly referred to as “bread-loafing”. Bread-loafing causes the depth of the layer of the filling material on the side walls of the trench adjacent the open mouth thereof to increase in a direction towards the other side wall at a greater rate than the rate at which the depth of the layer of the filling material on the remainder of the side wells below the open mouth increases. Accordingly, in due course as the filling of the trench progresses the bread-loafing of the filling material causes the layers of the filling material on the side walls adjacent the open mouth to meet and thus bridge the trench. Once the bread-loafing of the filling material has caused bridging of the trench, no further filling material can be filled into the trench, thus leading to the creation of a longitudinally extending void in the trench. Where the trench is to be entirely filled with deposited oxide the bread-loafing effect causes bridging of the trench with the oxide adjacent the open mouth of the trench. On the other hand, where the trench is to be lined with one or more layers of oxide, and then subsequently filled with polysilicon, the bread-loafing effect causes bridging of the trench with the polysilicon material.
Reference is now made to FIGS. 1 and 2 which are photomicrographs of different magnifications, which illustrate a cross-section of a silicon layer showing one trench formed in the silicon layer which demonstrate the effect of bread-loafing in the formation of a void in the trench. The silicon layer is indicated by the reference numeral 1 and the trench is indicated by the reference numeral 2. In this case the trench 2 was lined with an oxide layer 3 which was deposited on side walls 4, a base and on a surface 6 of the silicon layer 1. The bread-loafing effect of the oxide layers 3 can be seen at 7 adjacent the open mouth 8 of the trench 2. The bread-loafing effect of the oxide layers 3 at 7 has not caused the oxide layers 3 to bridge the trench 2. However, it has significantly narrowed the remainder of the open mouth 9 into the trench 2. A polysilicon layer 9 was then deposited over the oxide layer 3, and as can be seen the bread-loafing effect rapidly caused the polysilicon adjacent the open mouth 8 to bridge the trench. At that stage no further polysilicon could be deposited into the trench beyond the bridged open mouth 8, and thus, a void 10 extending longitudinally along the length of the trench was formed. As can be seen the void 10 extends from the open mouth downwardly towards the base. Furthermore, the void 10 terminates at the open mouth at a level substantially co-planar with the surface 6 of the silicon layer 1.
While, in general, the existence of such voids do not affect the electrical insulating characteristics of the filled trenches, they do lead to areas of entrapment where gases and liquids used in subsequent processing steps of the silicon layer can become entrapped, and subsequently lead to contamination of the silicon layer. Typically after the trenches have been filled the oxide and polysilicon layers are etched or ground back to the surface level of the silicon layer. As can be seen from FIGS. 1 and 2 etching or grinding the oxide and polysilicon layers 3 and 9 back to the surface 6 of the silicon layer 1 will remove the portion of the oxide and the polysilicon layers 3 and 9 which are bridging the trench, thus providing an opening to the void 10. Accordingly, if the silicon layer is cleaned by washing and rinsing, cleaning chemicals and/or rinsing water readily become entrapped in the void 10. Such cleaning chemicals, in general, would be contaminants in subsequent processing steps of the silicon layer, and on leaking out through the voids would lead to contamination in the subsequent processing steps which is unacceptable.
Another problem arises where a photoresist layer is to be formed on the surface 6 of the silicon layer through which voids have been exposed. In general, during the formation of a photoresist layer on a silicon or other layer the structure is spun in order to spread the photoresist layer over the surface on which the photoresist layer is to be formed. The spinning of the photoresist over areas where voids are exposed leads to uneven coating in the form of streaking of the photoresist over the surface of the silicon layer. This, thus, prevents accurate patterning of the photoresist layer. Additionally, the photoresist may become entrapped in the voids, thus leading to contamination in subsequent processing steps.
Indeed, in many cases after the filling of trenches in a trench isolated silicon layer, as well as the oxide and/or polysilicon or other such filling layers being etched or ground to the original surface of the silicon layer, the silicon layer is further ground to reduce the depth of the silicon layer. In such cases, even where a void formed in a trench does not extend up to the open mouth of the trench, subsequent grinding of the silicon layer, can lead to the removal of the bridging of the trench, thus exposing the void.
There is therefore a need for a method for forming a filled trench in a semiconductor layer of a semiconductor substrate in which the effect of trench voids is minimised. There is also a need for a semiconductor substrate with a semiconductor layer having filled trenches therein in which the effect of trench voids is minimised.
The present invention is directed towards providing such a method and a semiconductor substrate.